The long range goal of our research is to understand the molecular mechanisms of contractile activation in cardiac and skeletal muscle, how different isoforms of contractile and thin filament regulatory proteins manifest in different functional properties of these 2 striated muscle types, and how alterations in these proteins can lead to dysfunction in a variety of cardiomyopathies. Evidence from this grant, summarized in the Progress Report, indicates distinct differences in the cooperative activation of cardiac vs. skeletal muscle thin filaments during contraction. These differences may allow for greater cellular level control of force and explain the greater sarcomere length (SL) dependence of force in cardiac muscle that is required to match stroke volume with venous return on a beat-to-beat basis, ie. the Frank-Starling Law of the Heart. In this proposal we use parallel experiments of cardiac and skeletal muscle proteins and myocytes to study detailed mechanisms of the myofilament protein interactions involved in activation and modulation of contraction. Specific aims will investigate 1) how strong crossbridge binding affects Ca2+ binding to troponin 'locally', ie. within structural regulatory units (RU = 1 troponin, 1 tropomyosin, 7 actins), and between RUs in cardiac muscle; 2) the role of TnC Ca2+ binding and TnC-Tnl interaction properties in local RU activation and between RUs along thin filaments; 3) the role of myosin binding protein-C (MyBP-C) in cooperative activation, and 4) how TnC-Tnl interaction properties and MyBP-C influence the sarcomere length dependence of force development. An exciting aspect of this proposal is a novel 3D-myofilament half-sarcomere model that is spatially explicit, and contains the stochastic kinetics of cycling crossbridges and thin filament dynamics (aim 5). This model will aid in experimental design, analysis of results and understanding the multiplicity of protein interactions involved in thin filament activation and the SL dependence of contraction.